Vacuum tube



H. A. SNOW July 21, 1936.

VACUUM TUBE Original Filed March 19, 14930 2 Shee'ts-Sheet l ggd INVENTOR 'HAROLD A. snow BY NQ Q /LOt/*f/L/ ATTORNEY l l -aa -so fc5 raz rs H. A. sNoW July 21, 1936.

VACUUM TUBE Original Filed March 19, 1939 2 Sheets-Sheet 2 INVENTOR HAROLD A. SNOW y BY ff@ ATTORNEY .l Patented july 21, 1936 l PATENT orifice VACUUM vTUBE Harold A. Snow, Moun tain Lakes, N. .'i., assigner to Radio Corporation of America, a corporation of Delaware Original application March 19, 1930, Serial No. 437,225. Divided and this application July 2, 1931, Serial No. 548,295. Renewed December 7 Claims.

has imposed severe limitations upon the range` of signal voltages which may be applied to the 10 amplifier. Inradio receivers, for example, high sensitivity is desirable for the reception of Weak signals, and some form of manual, o'r automatic, control'must be provided'to reduce the amplier transmission, or gain, when stronger signals are l received. When a receiver of high sensitivity is operated in the vicinity of a broadcastingstation, it is not unusual to find that the signal voltage applied to the lrst carrier wave amplifier is greater than the Voltage required onthe detector for normal output at the loud speaker. With the present types of electron ydischarge tubes, it is usual to adjust one of the operating potentials applied to the tube electrodes to decrease the amplification as the received signal strength increases.

Within the range of relatively low `signal strengths, this reduction of amplification is not accompanied by modulation distortion, but' with increasing signal strengths distortion is introduced when the amplification rate is adjusted to maintain an approximately constant output. Furthermore, Within the range of higher signal strengths, it is frequently diflicult to adjust the amplification to maintain constant output since the transconductance of the tube changes very rapidly for small changes in the transmission control voltage. By transconductance is meant the ratio of the change in the current in the circuit of an electrode to the change in' the voltage on another clectrode, under the condition that all other voltages remain unchanged. This restricts the amplication control to a small range of applied control voltages, and, unfortunately, .the rate of change of amplification is more gradual in the range of high amplification where a rapid change A of amplification for small changes in control voltage would be permissible.

Cross-talk effects in radio frequency amplifiers depend upon the high-order curvature parameters of the tube, and are to that extent related to the problem of distortion discussed heretofore. It is pointed out that the term cross-talk" (Cl. Z50-27.5)

amplifier tubes by modulation between two, or more, signals. 'Ihe improvements discussed herein in connection with an electron discharge tube to reduce distortion will also reduce a. large part of the cross-talk. Reference is made to the Proceedings of the Institute of Radio Engineers for December, 1930, wherein in an article entitled Receivers by Means of Variable-Mu Tetrodes there is demonstrated the intimate relationship between the problems of distortion and cross-talk in radio frequency amplifiers, and their elimination by means of variable mu tubes.

It is, therefore, one of the main objects of the present invention to provide a space discharge tube haviiig such operating characteristics, that no distortion is introduced when, for increasing signal strengths, the operating potentials are so adjusted that the amplification rate is reduced to a small fraction of the maximum amplification.

A further object of the invention is to provide an electron discharge tube having such characteristics that, when the potentials are adjusted to give a. relatively low amplification of strong signals, the change in transconductance for a given change in the gain control voltage is much lower than is the case with the known types of tubes.

Another, and important object of the invention is to provide a high frequency amplier tube capable of adjustment to give an undistorted output of approximately constant magnitude over a Wide range of applied carrier voltages.

A more specific object of `the invention is to provide an electronic amplifier in which different portions of the electron stream are influenced at different rates by the voltages applied to the control grid.

The novel features which I believe to be characterlstic of my invention are set forth in particularity in the appended claims,.the invention itself, however, as to both its organization and method of operation will best be understood by referencepto the following description taken in connection with the drawings in which I have indicated diagrammatically several arrangements whereby my invention may be carried into effect.

In the drawings,

Fig. 1 is a perspective view, partly in section, of a screen grid tube showing an illustrative embodiment of the invention, i

Fig. 2 is a diagrammatic view illustrating another embcdiment of the invention,

Fig. 3 is a curve sheet showing the variations of plate current with grid bias for a tube such as Shown in Fig. 1,

Fig. 4 is a curve sheet showing the relation between control grid voltage and transconductance for tubes embodying the invention,

Fig. 5 is a curve sheet showing the relation between permissible maximum input voltages and control grid bias voltages, and

Fig. 6 is a curve sheetl showing the performance of a three stage amplifier employing the novel form of tube.

Referring to the accompanying drawings wherein like reference characters in the different gures designate the same elements, the invention is shown in Fig. l as embodied in a form of tube known commercially as a screen grid tube, the latter having a separate heater for the cathode. As is well known, this particular type of tube comprises an evacuated envelope enclosing a cathode C, heatedrby a resistance (not shown) within the cathode, an inner or control grid CG, a screen grid SG, a plate or anode P, and an outer screen S which is electrically connected to the screen grid. Except for the novel construction of the control grid CG and its novel functional relationships to the remaining elements of the tube, the several elements of the tube, and their relative physical arrangement, may be substantially theA same `as that employed in the present commercial tubes.

In vtubesoi' this general type, the control grid comprises a helical winding supported by one or more wires I. In this particular construction, the helical winding is not continuous, as in the known constructions, but comprises two sections 2, 2 that are separated by a distance of the order of twice the pitch of the winding. The windings of each section are of the same pitch, which may be the same as that now employed in tubes of this type. This particular construction `therefore physically differs from the known construction, of the same general physical design, by theabsence of two complete circumferential turns of the control grid winding.

This particular construction results in a tube erative, and the tube has about the same characteristics it would have if there were no gap in the grid. As the grid bias increases negatively the electron currents through the upper and lower parts ofthe control grid are cut off leaving a low-mu control through the gap. At these bias voltages the tube acts as if the upper and lower sections of the control gridwere formed of solid metal, and controlled the current through the gap in the ordinary manner, Y

The diagrammatic view of Fig. 2 shows a control grid CG which has threesection's a', b', c placed end to end and each of different pitch.

The mode of operation of the modification disclosed in Fig. 2 is dependent on the same general principle as has been explained in connection with the construction shown in Fig. 1. Essentially, and basically considered, the tube structures disclosed herein provide a radio frequency amplifier tube having a mii-factor which decreases continuously with increasing negative grid bias.

n is weu known that the ampuacancn of a vacuum `tube may be regulated by adjusting the bias voltage upon the control grid, the amplification decreasing as the bias voltage becomes more negative. Curves showing the relation between plate current and grid bias, i. e., transfer characteristics, afford an indication of the amplication at different bias voltages, since the slope of the curve at any point is a measure of the amplication when the tube is biased for operation at that point.

In Fig. 3, the solid line curveA is the transfer characteristic for a tube such as shown in Fig. l, and the dotted line curve B isa similar curve for a commercialscreen grid tube of the same general type but having a continuous control grid winding of uniform pitch. An examination of curve A shows that, with tubes embodying the invention, a control of amplification extends over a range of control grid bias of'from zero to morev than -30 volts. With the known tubes, the curvature of the transfer characteristic Y approaches zero at a control grid bias of about -15 volts.

In other words, an increase of the grid bias above approximately 15 volts negative will not be accompanied by a decrease in ampliiication when the known type of tube construction is employed, but with tubes embodying the invention, the ampliiication may be varied with changes of control grid bias throughout a range of from zero to upwardly of -30 volts. The tubes will still pass signals, by leakage transmission when the control grid biases exceed these respective values, but control of amplification is no longer possible in regions where the transconductance curves become substantially horizontal.

The curves of Fig. 4 show the relation between control grid bias and transconductance for two tubes embodying the invention, and for a similar tube which has the usual grid construction.

curve for a screen grid tube of the type shown in Fig. l, having two turns omitted from the center of the control grid. 'I'he data for this curve and for curve A' of Fig. 3 relates to theY same tube. Curve A' is a similar curve for a screen grid tube in which only one turn was removed from the center of the control grid, and curve B shows the characteristic properties of the conventional type of tube having a continuous control grid winding.

Curve A is the transconductance-control grid bias An examination oi' vthese curves shows that o occur when, for a given signalstrength, the amplication is so adjusted as to bring the output down to a desired, or standard, level. Since such distortion is due to the curvature of the transfer characteristic it will be vapparent that a tube having a curve of lower curvature can transmit, without distortion, higher voltage signals than a tube having a characteristic which exhibits a region of higher curvature. An examination of the curves of Fig. 3, will show that the maximum strength which may curvature of curve A is substantially lower than that of curve B.

Modulation distortion introduced by a tube may be determined by applying a signal having a definite and constant modulation to the input of the tube, and measuring the modulation of the output signal. For small input signals the tube introduces practically no change in modulation. When the input signal increases beyond a certain value the modulation of the output signal increases rapidly due tothe curvature in the tube transfer characteristic. This increase in modulation (modulation distortion) limits the maximum input signal that may be transmitted by the tube without distortion.

The curves of Fig. 5 show the relation between control grid bias voltages and the maximum input signal voltages which produce a4 twenty per cent rise in modulation. The twenty per cent rise in modulation was chosen as a standard as a matter of convenience since distortion ofthis magnitude may be observed by ear when the modulation is within the range of audible frequencies as is the case with speech or music. The data for curves A, A', and B was obtained for the same tubes `as those whose characteristic curves are identified by corresponding characters in Fig. 4, the signal in each instance being an 850 kilocycle carrier, modulated 30% at 60 cycles. k

In the case of the commercial tube, curves B of Figs. 4 and 5 show that over the range of bias voltages which control the amplification, i. e., from zero to about twelve volts negative, the maximum input signal voltage which can be transmitted with not more than 20% distortion vis about 5 volts.

When the bias is adjusted to maintain a coni stant output signal, the maximum input signal voltage that c an be applied to the tube with less than 20% distortion is about 0.3 volt, corresponding to a control grid bias of approximately 12 volts negative. For greater signal strengths, the increases of control grid bias do. not alter the amplification, but do affect the maximum signal be transmitted with' less than 20% distortion. `For a single tube, signal strengths falling outside the range of volume control cannot be handled by the amplifier if a constant output is essential, but in cascaded amplifiers having two or more controlled stages, the maximum voltages, as shown by the -dotted line v portion of curve B, may be transmitted by the first stage when amplification control without additional distortion is provided in a subsequent stage.

A similar analysis of curve A' will showthat the maximum signal strength which may, without undue distortion, be transmitted to give the desired constant output voltage is about 1.1 volts,

, corresponding to a control grid bias of about -224 type.

38 volts. For the tube with two turns removed from the control grid, curve A shows a permissible input voltage of 3 volts, with a bias of -95 volts. Furthermore, by so restricting the control grid bias voltages that the maximum can never exceed about 28 volts negative in the case of the tube lof curve A', and about 65 volts in the case of the tube of curve A, the maximum carrier voltages which may-be transmitted in approximately 7 and 22 volts, respectively.

The observations have been verified by tests made with a commercial radio receiver having three radio frequency amplifier stages employing commercial screen grid vacuum tubes of the Measured carrier wave voltages, 850

kilocycle modulated 30% vt 60 cycles, were impressed upon the first amplifier and the amplication was adjusted to maintain a constant carrier voltage on the detector which, for undistorted'carrier amplification, corresponded to a 60 cycle audio frequency output of 4 volts across the vspealrer terminals.

Modulation distortion inthe amplifier is evidenced by an increase in audio frequency output voltage when the carrier component of the detector input remains constant. By maintaining a constant carrier voltage at the detector the demodulated audio frequency output will remain constant `up to the point at which modulation distortion begins. Beyond that point, the audio frequency output will Arise even though the carrier voltage across the demodulator is maintained constant as the signal strength increases.

In Fig. 6, curve D shows the relationship between audio output and carrier wave input when commercial 224 tubes were used in the amplifier. To eliminate distortion in the third radio frequency stage, only the first two stages were adjusted to control the amplification. The distortion was of the same values whether the bias on the control grid of the first tube was varied, or the bias control was extended to include both the first and second tubes. From curve D it will be noted that the modulation rise begins at a carrier input of about 0.15 volt, and reaches 20% at 0.3 signal voltage on the first tube. i

Curve E was plotted from `data. obtained with the same radio receiver when tubes embodying the invention were substituted in the radio frei quency stages.

in Fig. 1, i. e., of standard 224 type construction The tubes were of the type shown except that two turns were omitted from the center of the control grid. With increasing signal strength, the detector input was maintained constant by adjusting the control grid bias simul- I taneously on the three amplifier stages. It is.to f be noted that the carrier input across the first tube increased to 10 volts before a modulation rise wasapparent, and that it reached 17 volts before the modulation rise reached 20%. The

, variation of control grid bias with input signal Y the input on the first amplifier was from l5 to 20 volts, becoming worse as beyond 20 volts.

yThese observations of actual performance in a the input was increased receiver check closely with the results plotted in Figs. 4 and 5 for single stages. By employing tubes constructed in accordance with the invention, the permissible input voltage was raised from 0.3 volt to 17 volts, i. e., volume control with good quality reproduction can be had with input voltages about 57 times as great as those which may be applied when the known.comrner cial form of tube is employed.

AItis to be understood that the invention is not limited to any particular type of tube, but is, in generaLapplicable to all tubes employed for amplification control. The physical construction of the control grid, or the geometric and structural relationships of the tube elements are subject I to wide variation so long as the control grid exercises different rates of control at different portions of the electron stream. Considered V lio ing a relatively high ratio of plate voltage to control grid voltage (high mu) one or more, of the remaining tubes having lower ratios of plate voltage to control grid voltage (low mu). In fact, the same distortionless amplification control and substantial reduction of cross-talk over a wide range of input voltages may be secured when two or more tubes of the described different characteristics are operated in parallel. A single tube exhibiting these characteristics will usually be more economical and convenient than the' parallel tube arrangement. Y

vAlthough the above discussion has been limited to a consideration o1 modulation distortion in radio receivers, it will be apparent that the curvature of the transfer characteristic gives rise to other forms of distortion which limit the range oi continuous wave and audio frequency voltages within which a tube acts as a substantially linear amplifier. The invention provides a means for extending the range of signal voltages which may be transmitted without distortion, the signals being either faudio or 'radio"freer1uency,i

and if of radio frequency, either continuous wave or modulated.

While I have indicated and described several systems for carrying my invention into effect, it

will be apparent to one skilled in the art 'that my invention is by no means limited to the particular constructions shown and described, but

that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. An electrondischarge device comprising an elongated unipotential cathode, a plate surrounding said cathode, and a unipotential control grid surrounding and coextensive with said cathode and having at least three helical sections of the same diameter mounted end to end and each covering a substantial portion of said cathode, each f said sections having a plurality of turns and being of a uniformpitch which differs from the pitch of the adjoining section, thepitches of said sections being so related that the plate current varies in substantially exponential relation to variation in voltage on said control grid.

2. An electron discharge device comprising an equipotential cathode. a coaxial anode, and an interposed coaxial unipotential control grid consisting of three helical sections of the same diameter mounted end to end, each sectionA covering one-third of the length of said cathode and having a plurality of turns and being of vuniform pitch and the pitch of each section being substantially different from the pitch of any other section for influencing at substantially different rates those portions of the electron stream from said cathode to said anode which flow through said different sections'of said control grid.

3. An electron discharge tube having a. variable mu factor and comprising an equipotential cathode, a 'coaxial anode, and a unipotential control grid coaxial withsaid cathode and comprising three sections of uniform diameter-.and mounted end to end, each of said sections havrates of control on those ing a plurality of turns and being of a uniform pitch which differs from the pitch of the adjoining section and bears to the pitches of the other sections a relation` such that' the grid voltage anode current characteristic of the tube is substantially exponential and has a remote outoi point on the negative grid voltage side.

4. A vacuum tube comprising an equipotential vcathode and a plate, a multisection control grid positioned between said cathode and said plate and consisting of a plurality of sections mounted end to end, each section having a uniform pitch differing from the pitch of any othersection to exercise at different portions of vthe electron stream between said cathode and said plate rates of control which differ progressively by amounts which cause the relation between variations of grid voltage and plate current to be substantially exponential, and a screen grid between the control grid and the plate.

5. An electron discharge tube having a variable mu factor and comprising a cathode, an anode coaxial with said cathode, and a unipotential grid electrode coaxial with said cathodeand interposed between said cathode and said anode, said grid electrode comprising a plurality of adjoining sections each having a plurality of conductors uniformly spaced to permit an electron stream to fiow from said cathode and between said conductors to said anode, thespacing of the conductors in each of said adjoining sections being suiciently different to cause each of said sections to exert substantially different portions of the electron stream from said cathode to 'said vanode which i'iow through said different sections of said grid electrode.

I 6. An electron discharge tube comprising a cathode, an anode, and a unipotential grid electrode between said cathode and said anode cony sisting of a plurality of sections each having uniformly spaced conductors, the spacing of the conductors in each section differing from the spacing of the conductors'in adjoining sections by amounts which cause the different portions of an electron stream flowing from said cathode to said anode through said different sections to be reduced to a minimum in succession by different sections of said grid electrode as the potential of saidgrid electrode is made progressively more negative with reference to said cathode.

'7. A unipotential helical grid electrode for electron discharge devices comprising at least three helical windings of the same diameter mounted end to end with all their turns connected in parallel,` each of said sections being of uniform pitch and having a plurality of turns, the pitch of each of said sections differing from -the pitch of an adjoining section by an amount HAROLD A. sNoW. 

